1. Field of the Invention
This invention relates, in general, to bioelectrical impedance measurement apparatus and methods and, more specifically, to bioelectrical impedance measurement apparatus and methods using an impedance plethysmograph.
2. State of the Art
Bioelectrical impedance measurements have been employed to determine various body characteristics, such as blood flow, cardiac output and composition including an assessment of body fat, lean body mass and extracellular mass. To determine body composition, a four-electrode impedance plethysmograph is usually employed. A first pair of source or current electrodes is connected to a human body typically on a hand and a foot. Specifically, one source electrode is attached to the dorsal surface of a hand over the metacarpals, whereas the other source electrode is attached to the distal end of the third metatarsal bone. One electrode of a second pair of detecting or sensing electrodes is traditionally attached to the dorsal surface of a hand between the bony prominences of the wrist, whereas a second sensing electrode is positioned between the lateral and medial bony prominence of an ankle. An excitation current generated by the plethysmograph is applied to the source electrodes and thus introduced into the body. For example, an 800 microamphere, 50 kHz current is typically employed.
The human body opposes the conduction of electrical current and this ability to oppose current is called impedance. Impedance can be measured by the plethysmograph and analyzed into a resistance and reactance value. The whole body resistance is then combined in an equation with the weight and height of the subject to predict total body water (TBW). When TBW is predicted according to this technique, a reasonably high correlation is present with TBW volume determined from the "golden standard", namely, heavy water dilutional technique.
In the measurement technique described above, the arm, trunk and leg of the subject are connected in a serial fashion. However, the forearm and lower leg are body parts with small diameters in comparison to the upper arm, thigh and trunk. When the resistance is determined across individual parts of the human body, more than 50% of the whole body resistance signal is present in a forearm and a lower leg, although the contribution of these anatomical parts to body composition is less than 5%.
Thus, it would be desirable to provide a bioelectrical impedance measurement method and apparatus which increases the accuracy of a single body compositional measurement of a subject. It would also be desirable to provide a bioelectrical impedance measurement method and apparatus which provides body impedance measurements which are superior in their prediction of total body water (TBW) when compared to the conventional (whole body impedance) method. It would also be desirable to provide a bioelectrical impedance measurement method and apparatus which excludes the forearm and lower leg of a subject from the measurement so as to increase the accuracy of TBW prediction. Finally, it would be desirable to provide a bioelectrical impedance measurement method and apparatus which generates highly accurate body impedance measurements which may be used to improve fluid management of ill subjects in a clinical setting, such as in an intensive care unit.